This invention relates to a process for imaging a lithographic printing plate, and more particularly to a process for using an ink jet printer to imagewise apply an insolublizing chemical to a lithographic printing plate having a coating containing diazo compounds.
In the art of lithographic printing it is generally required that one or more lithographic printing plates be mounted on a printing press. The lithographic printing plate is characterized by having on its printing surface oleophilic ink receiving areas in the form of the image to be printed, and hydrophilic water receiving areas corresponding to the other, non-printing areas of the surface. Because of the immiscibility of oil-based lithographic inks and water, on a well-prepared printing plate, ink will fully coat the oleophilic areas of the plate printing surface and not contaminate the hydrophilic areas. The operating press brings the inked plate surface into intimate contact with an impression cylinder or elastic transfer blanket that transfers the ink image to the media to be printed.
Traditionally, a lithographic plate is photographically imaged. The plate substrate is most commonly aluminum, treated so that the printing surface is hydrophilic, although treated or untreated plastic or paper substrates can also be used. The hydrophilic substrate is then coated with one or more layers of polymer or resin solutions. The deposited coating is generally oleophilic, although the coating solution may be aqueous-based or solvent-based. A coating deposited from an aqueous-based coating solution is known as aqueous-borne; from a solvent-based solution, solvent-borne. Coating layer thickness are commonly about 1 to 3 microns thick.
At least one of the layers of the plate coating is photosensitive. Light sensitive coating compositions for lithographic printing plates are well known in the art as taught in U.S. Pat. Nos. 4,511,640; 4,917,988; 3,785,825; 4,186,069; 4,224,398; 4,273,851; 4,288,520; 4,299,907 and 5,688,627, and are incorporated herein by reference. The photosensitive layer most commonly comprises diazo resins. Diazo resin coatings can be prepared such that the diazo functional groups undergo photochemically initiated cross-linking reactions on exposure to light typically having wavelengths from 325 to 400 nm. The photochemical reaction products are generally acidic, effectively reducing the pH in the imaged area of the coating. Alternative photosensitive layers comprise diazo compounds mixed with non-photosensitive polymers or resins, or other photosensitive polymers without diazo compounds. The plate thus prepared is considered photographically presensitized (PS).
Even when other photochemically-active polymers are used to impart photosensitivity to the coating, some amount of diazo resin may be added to the coating solution to promote adhesion between the coating and the aluminum surface or to act as a binder for the coating. Such a plate would be considered presensitized based on the photosensitizing polymers contained even if the diazo compounds contained were not in themselves sufficient to impart imaging capability.
To prepare a PS plate for printing, the plate is first exposed to light in the pattern to be printed using a film negative. The exposed plate is then washed in a developing solution. The exposed areas of the plate coating are insoluble; the unexposed areas are dissolved and quantitatively removed from the hydrophilic aluminum surface of the plate substrate. Such a preparation process is referred to as a negative working process because the unexposed coating is removed. In a positive working process, the pattern to be printed is masked and the photosensitive exposed coating is rendered soluble in a developer. Until after the development step, the printing artisan or press operator generally endeavors to not allow incidental exposure of the plate to typical white light or sunlight. Undeveloped plates are typically only handled in low light or "yellow light" rooms or conditions.
The insolubility of the exposed coating is typically caused by photochemically induced cross-linking of the diazo resins. Plates relying on photopolymers comprising photosensitive functional groups other than diazo functional group may be oxygen sensitive. In such a case, the rate of cross-linking may be enhanced by an oxygen inhibition barrier layer over the photosensitive layer.
Lithographic printing plates generally have images that are planographic, i.e., substantially flat. But other printing plates with similar photosensitive coatings may have raised images for relief printing or intaglio images for gravure printing. Lithographic printing processes may use water as described above, or they may use a waterless printing technique. If a waterless technique is used, then the discrimination between the inked and non-inked areas of the plate surface is based on having different surface energies in the coated and non-coated areas.
Traditionally, lithographic plates are imaged by photographic transfer from original artwork. This process is labor-intensive and costly. Hence with the advent of the computer engendering a revolution in the graphics design process preparatory to printing, there have been extensive efforts to pattern printing plates, in particular lithographic printing plates, directly using a computer-controlled apparatus such as a platesetter which is supplied with digital data corresponding to the image to be printed. A platesetter has the capability to supply an image forming agent, typically light energy or one or more chemicals, to a plate according to various images as defined by digital data, i.e., to imagewise apply an image forming agent. Specially manufactured lithographic plates may be required for certain types of platesetters. Such a combination of a computer-controlled platesetter and the proprietary plates used with them along with developer solutions and any other materials or apparatuses necessary to prepare the plates for printing is known as a computer-to-plate (CTP) system.
Heretofore, many of the new CTP systems have been large, complex, and expensive. They are designed for use by large printing companies as a means to streamline the prepress process of their printing operations and to take advantage of the rapid exchange and response to the digital information of graphic designs provided by their customers. There remains a strong need for an economical and efficient CTP system for the many smaller printers who utilize lithographic printing.
Many of the new CTP systems use light sources, typically lasers, to directly image PS plates. But using lasers to image plates is very expensive, because the per-unit cost of the lasers is high and because they require sophisticated focusing optics and electronic controls. If because of the cost only a single laser is used, then time becomes a constraint because of the necessity of raster scanning.
In recent years, ink jet printers have replaced laser printers as the most popular hard copy output printers for computers. Ink jet printers have several competitive advantages over laser printers. One advantage is that it is possible to manufacture an array of 10's or even 100's of ink jet nozzles spaced very closely together in a single inexpensive printhead. This nozzle array manufacturing capability enables fast printing ink jet devices to be manufactured at a much lower cost than laser printers requiring arrays of lasers. And the precision with which such a nozzle array can be manufactured and the jetting reliability of the incorporated nozzles means that these arrays can be used to print high quality images comparable to photo or laser imaging techniques. Ink jet printers are increasingly being used for prepress proofing and other graphic arts applications requiring very high quality hard copy output. In spite of the large and rapidly growing installed base of ink jet printers for hard copy output, ink jet printing technology is not commonly used in CTP systems. There are many challenging technical requirements facing the practitioner who would design such an ink jet based CTP system as can be seen in the prior art.
A first requirement is that the ink jet ink used to image the printing plate be jettable, able to form ink drops of repeatable volume and in an unvarying direction. Further, for practical commercial application, the ink must have a long shelf life, in excess of one year or more. U.S. Pat. No. 5,970,873 (DeBoer et al) describes the jetting of a mixture of a sol precursor in a liquid to a suitably prepared printing substrate. But any ink constituents of limited solubility will render unlikely the practical formulation of a jettable, shelf-stable ink. Similar problems exist in U.S. Pat. No. 5,820,932 (Hallman et al) in which complex organic resins are jetted, and U.S. Pat. No. 5,738,013 (Kellet) in which marginally stable transition metal complexes are jetted.
Another requirement is that to be of wide utility, the ink jet based CTP system be able to prepare printing plates with small printing dots, approximately 75 microns in diameter or smaller, so that high resolution images can be printed. Ink jet printers can produce such small dots, but of those having substantial commercial acceptance, only ink jet printers employing aqueous-based inks are practically capable of printing such small dots. Thus the systems described in U.S. Pat. Nos. 4,003,312 (Gunther), 5,495,803 (Gerber), 6,104,931 (Fromson et al), and 6,019,045 (Kato) which use solvent-based hot melt inks will not allow the preparation of the high resolution printing plates necessary for printed images of high quality. It is also required that the prepared printing plates be rugged, capable of sustaining press runs of many thousands of impressions. The waxes used in the hot melt inks described in U.S. Pat. No. 6,019,045 (Kato) and 4833486 (Zerillo) would wear out in such a long press run.
Another requirement of a successful ink jet based CTP system is that a mature plate technology is to be preferred. There are many tradeoffs in the manufacture of commercially practical lithographic plates. They must be highly sensitive to the imaging process and yet thermally stable, stable in high humidity storage environments and yellow light, resistant to fingerprints, of minimal toxicity and environmentally benign, easily developed in that small dots are quantitatively resolved without dot blooming using developers that are of minimal toxicity and environmentally benign, able to sustain long press runs, manufacturable at a low cost per square foot, and many other practical requirements. U.S. Pat. No. 5,695,908 (Furukawa) describes a process for preparing a printing plate comprising a new plate coating containing a water-soluble polymer that becomes water-insoluble in contact with a metal ion in a solution jetted imagewise. But such a new plate coating is unlikely to meet the wide array of constraints on a successful plate technology. U.S. Pat. No. 6,025,022 (Matzinger) describes a new plate coating on a glass substrate that would be unlikely to find wide acceptance.